Optical olfactory sensor with holographic readout

ABSTRACT

This invention relates to optical detection of vapors, in particular devices and methods for detection of vapor concentration and changes in vapor concentration using dynamic holography. The devices and methods employ a transducer which absorbs the vapor to be tested, thereby leading to a change in the transducer. The changes in the transducer cause a change in the optical path length of an image beam which is interacted with the transducer. Dynamic holography allows determination of the change in the dimensions and index of refraction of the transducer, and thus the change in the concentration of the vapor to be tested. The devices and methods of the invention are capable of testing a plurality of vapors by using a transducer array.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/394,490 filed Jul. 8, 2002, which is hereby incorporated by referencein its entirety to the extent not inconsistent with the disclosureherewith.

BACKGROUND OF THE INVENTION

This invention relates to optical detection of vapors, in particulardevices and methods for detection of vapor concentration and changes invapor concentration using dynamic holography.

Vapor detection devices exist in a variety of forms. One form of vapordetection device employs a transducer to detect changes induced by thevapor, rather than analyzing the vapor directly. The transducer may behighly selective towards an individual vapor (“lock and key” approach).Alternatively, the transducer may respond to several vapors and an arrayof different transducers may be used to produce a “signature” which isused to classify, and in some cases quantify the vapor of concern(Severin et al. (2000), Anal. Chem. 72, 658-668). Vapor detectiondevices employing transducers have a variety of commercial, industrialand military applications.

In particular, vapor detection devices employing transducers have beenused for olfactory sensors, also known as artificial or electronicnoses. An artificial nose typically contains an array of dissimilartransducers simulating the human olfactory response (Nagle, H. et al.,(September 1998), IEEE Spectrum, 22-34). Olfactory sensors have usedsurface acoustic wave (SAW), electrochemical, conducting polymer,piezoelectric, and optical methods for generating and detecting thetransducer response (White et al., (1996), Anal. Chem. 66, 2191-2202).SAW arrays have been limited in size because of the electroniccomplexity involved and the challenges associated withmicromanufacturing large numbers of such systems into an integratedsystem (Lonergan et al., (1996), Chem. Mater. 8, 2298-2312).

Many optical transducer-based vapor detection devices employ opticalfibers or other media for transmission of light through total internalreflection (e.g. capillary tubes). These devices have been configured ina variety of ways. For example, in intrinsic optical fiber sensors achange in the optical fiber itself occurs, while in extrinsic sensorsthe optical fiber serves as a conduit to transport light to and from thesensing element (Sietz., W. (1988), CRC Crit. Rev. Anal. Chem., 19 (2),135-173).

Fiber-optic sensors often consist of an analyte sensing elementdeposited at the distal end of an optical fiber, with the opticalsensing element typically composed of a reagent phase immobilized at thefiber tip by either physical entrapment or chemical binding. Thisreagent phase usually contains a chemical indicator that experiencessome change in optical properties upon interaction with the analyte(White et al., (1996), supra). Fluorescent dyes have been used aschemical indicators (White et al., (1996), supra; Oreliana, G. et al.(1995), 67, 2231-2238). A sensor or transducer array is made by usingmultiple fibers.

Interferometric fiber-optic sensors have also been constructed whicheffectively provide a single transducer or sensing element rather thanan array of transducers. The optical fiber is used to construct areference branch and a measuring branch of an interferometer. Themeasuring branch contains a sensing element which interacts with themeasuring branch, causing its optical properties to change so that thereis a shift in the phase of the transmitted light. When the light fromthe two branches is recombined, interference results (Sietz, W. (1988),supra). One interference sensor to measure the partial pressure ofhydrogen used a coating of palladium on the outside of the optical fiberfor a transducer. The higher the partial pressure of hydrogen, the morewas adsorbed in the palladium. This constricted the fiber and modifiedthe phase of light transmitted through the fiber (Butler, M. (1984),Appl. Phys. Lett., 45(10), 1007-1009). Another interference sensor (Valiet al., U.S. Pat. No. 5,004,914, issued Apr. 2, 1991) bonded thereference branch and measuring branch optical fibers tomagnetorestrictive substrates. The measuring branch substrate was coatedto facilitate collection of the vapor molecules. The frequency ofoscillation of the measuring branch substrate changed slightly inresponse to the collection thereon of molecules of the chemical vapor tobe detected, allowing a difference in resonant frequency between thereference and sensor substrates to be detected.

BRIEF SUMMARY OF THE INVENTION

The present invention provides interferometric vapor detection deviceswith a holographic readout which can be used as olfactory sensors.Embodiments of the devices have the following advantages: easilymanufactured transducer array, versatile response, repeatable response,fast response (within 5 seconds), and high sensitivity. The sensitivityof the devices depends upon the transducer material, but a sensitivityhas been attained for ethanol vapor of approximately 60 ppbmm²/sqrt(Hz).

Embodiments of the invention also provide methods for detection of vaporconcentration and changes in vapor concentration using dynamicholography. The methods analyze a dynamic signal rather than a DC(steady state) signal. As a result, the methods are insensitive toslowly varying environmental parameters. Furthermore, the signal tonoise ratio of the dynamic signal can be improved via filtering over anequivalent DC signal.

Additional embodiments of the invention provide devices and methods foroptical detection of changes in vapor concentration using dynamicholography. The vapor being detected is termed a “test” vapor. Themethods of the invention can detect a change from an undetectable testvapor level to a detectable test vapor level or from one detectable testvapor level to another. The methods are also capable of simultaneouslydetecting changes in concentration for multiple test vapors.

The methods of the invention can utilize a transducer capable ofabsorbing the test vapor. Changes in test vapor concentration can causechanges in the transducer's dimensions, changes in the transducer'sindex of refraction and/or other changes that can be detected opticallyusing dynamic holography. Multiple transducers, each of which respondsto a different test vapor, can be used simultaneously to detect changesin concentration for multiple test vapors.

The changes in the transducer are detected optically. In particular, thetransducer is placed in the path of a beam of coherent light, which isreferred to as the image beam. After the image beam interacts with thesample, it is used to generate an interference pattern. Changes in thedimensions and the index of refraction of the transducer cause changesin the optical path length and the intensity of the beam and thuschanges in the interference pattern. The amount of change in the opticalpath length of the beam indicates the amount of change in test vaporconcentration.

As used herein, dynamic holography involves generation of aninterference pattern, generation of a hologram based on the interferencepattern using a dynamic holographic medium, and reading out the hologramgenerated. Dynamic holography is used to provide a holographic readoutbased on the interference pattern and thereby determine the change indimensions and index of refraction of the transducer. The holographicreadout provides real-time information about changes in vaporconcentration.

As defined herein, a hologram is a record of the interference patternbetween two or more electromagnetic waves embodied by the spatialvariation of the dielectric constant, or index-of-refraction, of amedium or media. “Dynamic holography” is holography that involves eithera dynamic holographic medium (or media) or involves an apparatus,electronic or otherwise, that replicates the functionality of a dynamicholographic medium (or media) (e.g. digital holography). A “dynamicholographic medium” is a medium that is capable of performingholographic recording or readout nearly simultaneously on a nearlycontinual basis without substantial depletion or degradation of itsholographic properties over times of interest. “Holographic recording”is the process of producing a hologram using the interference ofelectromagnetic waves to itself lead to the index-of-refraction ordielectric constant variation in a recording medium (even if therecording medium requires additional elements and/or processing toeffect the index-of-refraction or dielectric constant variation). Fornon-digital holography, “holographic readout” or “reading out thehologram” is scattering of an electromagnetic wave from a hologram(usually in such a way as to reproduce a version of one or more of theoriginal recording waves). For example, holographic readout of ahologram can be used to reproduce a version of the original image wave.The term “holographic readout” can also be used as a noun referring tothe result of scattering of an electromagnetic wave from a hologram (forexample, the reproduced version of the original image wave). For digitalholography, “reading out” the hologram can involve reading out theinterference pattern information from a spatial information recordingdevice and processing the information recorded.

Embodiments of the invention also provide a method for determining theconcentration of a test vapor that is not necessarily changing. In thismethod, a reference vapor and the test vapor can be alternately suppliedto the transducer, creating a change in the vapor environment seen bythe transducer, which can be detected and analyzed using the methodsdescribed above.

Embodiments of the invention also provides a method for the detection ofa change in concentration of a test absorbant in a liquid environmentcomprising the steps of: providing a transducer capable of absorbing thetest absorbant and thereby changing the transducer; exposing thetransducer to the test absorbant; and detecting the change in thetransducer using dynamic holography, thereby detecting the change inconcentration of the absorbant. The change in the test absorbantconcentration can cause changes in the transducer's dimensions, thetransducer's index of refraction and/or other changes that can bedetected optically using dynamic holography.

The devices of the invention can employ one or more of the methods ofthe invention. The devices preferably have a real time response, withmeasurements being typically completed in less than 5 seconds andpreferably in less than 2 seconds. The devices can be operated withbattery power and can be made portable. By a portable device, it ismeant that the device is suitcase-sized, briefcase sized, or smaller.The devices of the invention have commercial, industrial, medical, lawenforcement and military applications. These applications includedetecting leaks in an industrial environment, monitoring a manufacturingprocess vapor environment (including pharmaceutical and cosmeticsprocesses), vapor recognition and tracking, and detecting biohazards,automobile emissions, chemical vapors associated with explosives,alcohol, controlled substances, spoiled perishable products, and toxicgases, to name a few.

In one embodiment, the devices of the invention are based on an opticalnovelty filter which incorporates a photorefractive element. A “noveltyfilter” shows what is new in an input image compared with the input'srecent history (Anderson and Feinberg, (1989), IEEE J. QuantumElectron., 25(3) 635-640, hereby incorporated by reference). Becausedevices based on novelty filters detect relatively rapid changes, thedevices are insensitive to slowly varying environmental parameters liketemperature, pressure and humidity. The novelty-filter based devices arealso self-adaptive to distortions in wave fronts and drifts in opticalpath.

Embodiments of the invention provide an olfactory sensor system with aholographic readout employing an optical novelty filter. This sensorsystem produces a change in the intensity of the transducer image at adetector when the test vapor concentration changes. In a two beamcoupling device, a reference beam and an image beam are combined withina photorefractive element such as a photorefractive crystal, thuscreating a hologram in the element. The output from the photorefractiveelement in the direction of the image beam consists of the image beamand a diffracted portion of the reference beam (the diffracted portionof the reference beam can be regarded as part of the holographicreadout). At steady state, the diffracted portion of the reference beaminterferes with the image beam to produce an intensity pattern at adetector placed after the photorefractive element in the path of theimage beam. If the vapor concentration and path length of the image beamchange suddenly, the phase difference between the image beam and thereference beam changes and the intensity of the transducer image at thedetector changes. Holographic optical novelty filter versions other thanthe two-beam coupling version described here can also be used, includingthose that do not require an externally-supplied reference beam, such asthose that make use of beam-fanning (amplified spontaneous scattering),and those that use self-pumped phase-conjugation (Anderson and Feinberg,(1989), supra and Ford et al., (1988), Optics Letters, 13(10), 856-858,hereby incorporated by reference).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a two-beam olfactory sensor system.

FIG. 2 schematically illustrates the diffraction of the reference beamby the internal refraction index grating generated by thephotorefractive element.

FIG. 3 schematically illustrates the response of four transducers on asubstrate, two sensitive to methane and two sensitive to hexane, to asudden increase in methane concentration.

FIG. 4 shows the relationship between the detector reading andconcentration for ethanol vapor during calibration of a two-beam coupledsensor system.

FIGS. 5A-5C show the response pattern to ethanol (5A), the responsepattern to hexane (5B), and the response pattern to a mixture of ethanoland hexane (5C) of a 2 by 2 transducer array.

FIG. 6 shows the response of a sensor system to changes in concentrationof the vapor environment.

FIG. 7 shows the relation between the sensor system sensitivity and theactive area of the transducer material.

FIG. 8 shows the relationship between the detector reading andconcentration for ethanol vapor for a two-beam sensor system using phasemodulation.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention provide an olfactory sensor system fordetecting changes in test vapor concentration in an environment. In oneembodiment, the sensor system comprises a coherent light source capableof producing a beam of light, a transducer in fluid communication withthe environment and capable of responding to a change in test vaporconcentration, a dynamic holographic medium, and a detector, wherein atleast part of the beam of light passes from the light source to thetransducer, from the transducer to the dynamic holographic medium, andfrom the dynamic holographic medium to the detector.

Depending on the desired sensor system configuration, the sensor systemcan additionally comprise a variety of elements. For example, a beamsplitter can be used to generate a reference beam and an image beamwhich are recombined in the photorefractive element as in a two-beamcoupling novelty filter. As is known by those skilled in the art,alternative novelty filter configurations are available which do notrequire that the beam be split (Anderson and Feinberg, (1989), supra;Ford et al. (1998) supra). Beam directing elements such as mirrors andprisms can also be placed in the beam path. Beam shaping elements suchas lenses, curved mirrors, filters, apertures, line generators, andstatic holographic elements can be used to change the beam diameterand/or to change the beam shape and/or tailor its intensity. Lenses canalso be used for beam imaging. As is known by those skilled in the art,the need for beam directing and shaping elements depends upon theparticular sensor system configuration and different beam directing andbeam shaping elements can be substituted for one another. Polarizationmodifying elements such as polarizers and half wave plates can be usedto adjust the polarization of a beam so that it is optimal for aparticular orientation of photorefractive element, as well as to producea variable beam splitter. The transducer may be supported on a substrateand a vapor feeding system can be used to control the vapor environmentin fluid communication with the transducer. Control systems can be usedto control the sampling rate of the detector and the output from thedetector can be fed to an analysis system for further processing.

In one configuration, transducer material can be applied to the outsideof an optical fiber, either on one end or along the length of a portionof a fiber where the fiber core is sufficiently close to the surfacethat an evanescent field is present in the transducer material. Severalsuch fibers can be used with different transducer materials for chemicalvapor sensing diversity. In such a case, the output of the fiber orfibers collectively serves as the image beam.

FIG. 1 shows a two-beam coupling system in which the beam from coherentlight source (10) is split by beam splitter (12) into an image beam(unshaded beam in FIG. 1) and a reference beam (shaded beam in FIG. 1).Beam-shaping elements (14-16), shown as lenses, increase the beamdiameter and change the beam shape. Beam-directing element (17), shownas a prism, is used to direct the image beam towards transducer (20).Transducer (20) is shown attached to substrate (22). Vapor feedingsystem (30) controls the environment to which the transducer (20) isexposed. Labels 150 and 151, respectively, indicate vapor being fed intoand out of vapor feeding system (30). If beam-directing element (17) isa prism, preferably the index of refraction of substrate (22) and prism(17) are matched and a layer of index-matching fluid (not shown) isplaced at the substrate-prism interface. At least part of the image beaminteracts by reflection and/or transmission, and/or evanescently withthe transducer then travels from the transducer to the photorefractiveelement (50). Optional lens(es) (18) keeps the image beam within thephotorefractive element (50) to optimize the dynamic holographicperformance. Optional polarizer (19), shown in the path of the imagebeam is used to align the polarization of the image beam with theoptical axis of the photorefractive element. At least part of the imagebeam entering the photorefractive element passes through the element asshown (and typically another portion will diffract) and travels todetector (60).

The reference beam is directed to photorefractive element (50) by beamdirecting element (70), shown as a mirror. An optional half wave plate(71) and polarizer (73) are used to align the polarization of thereference beam with the optical axis of the photorefractive element.Alternatively one can cut and orient a photorefractive crystal asdesired. Typically the reference beam enters photorefractive element(50) at an angle with respect to the image beam. Within thephotorefractive element, at least part of the reference beam isdiffracted in the direction of the image beam and so passes to detector(60).

FIG. 2 schematically illustrates the diffraction of the reference beam(beam 1) by the internal refraction index grating (100) generated by thephotorefractive element. In FIG. 2, the diffracted reference beam isbeam 1′ and the image beam is beam 2. The interference pattern (95)between beams 1 and 2 is also illustrated. For some photorefractivematerials at steady state, the portion of the reference beam diffractedin the direction of the image beam is π (180°) out of phase with theimage beam and destructively interferes with the image beam, producing alow (or null) intensity image of the transducer at the detector. Thegenerated grating has a π (180°) phase shift from the interferencepattern. In other materials the reference beam diffracts with adifferent steady-state phase. In any case, some pattern of intensity isproduced at the detector in steady-state. If the transducer responds toa sudden change in chemical vapor environment, the optical path lengthexperienced by the image beam will change. Until the photorefractiveelement adapts to the change in optical path length of the image beam,the phase difference between the image beam and the reference beam willno longer be its steady-state value and the transducer image at thedetector will be changed in intensity over the steady state transducerimage. Analysis of the information from the detector allowsdetermination of the change in dimensions and index of refraction of thetransducer.

FIG. 3 schematically illustrates the response of four transducers (20 aand 20 b) on a substrate (22), two sensitive to methane (20 b) and twosensitive to hexane (20 a), to a sudden increase in methaneconcentration. In the output image (80) the two transducers sensitive tomethane become bright while the two transducers sensitive to hexaneremain dark.

The transducer (20) is capable of responding to a change in test vaporconcentration by absorbing the vapor and producing an opticallydetectable change. For example, the change in transducer dimensions canlead to a change in optical path length, while the change in index ofrefraction can lead to a change in both optical path length and beamintensity. The desired transducer area depends upon the sensitivityrequired, with larger transducer areas giving higher sensitivity. Atransducer may be supported on one or more substrates. Forconfigurations where the image beam passes through the substrate beforereaching the transducer, the substrate is selected so that it does notsignificantly absorb the image beam and so that it does not respond tothe test vapor. In the configuration shown in FIG. 1, the substrate isselected to have an index of refraction close to that of the prism (forexample, a glass slide). Adhesive materials may be used to jointransducers to a substrate or to join substrates to one another.

Films of material are preferred for use as transducers. Polymer filmsare suitable for use as transducers, although other inorganic andorganic materials, including biomaterials such as proteins and enzymes,can be used. The polymer film can be doped with another material, suchas a metal, to increase the sensitivity of the transducer. In one modeof operation, each transducer material is selected so that itinteracts/absorbs with only a specific variety of vapors. This allowsfabrication of an array of transducer elements on a substrate, withdifferent transducers being used to absorb different test vapors. Thenumber of transducers selected is determined by the application, butarrays of 25, 50, 75, 100 or more transducers can be fabricated.

For polymer transducers, the solubility of test vapors in variouspolymers differs greatly. Therefore, it is preferable to use polymericmaterials that exhibit maximum response to the vapor(s) to be detected.Polymers known to the art of vapor sensors include, without limitation,poly(N-vinylpyrrolidone); poly(ethylene-co-vinyl acetate);poly(4-vinylphenol); poly(styrene-co-allyl alcohol);poly(α-methylstyrene); poly(vinyl chloride-co-vinyl acetate); poly(vinylacetate); poly(methyl vinyl ether-co-maleic anhydride); poly(bisphenolA-carbonate); poly(styrene); poly(styrene-co-maleic anhydride);poly(vinyl butyral); poly(sulfone); poly(methyl methacrylate);poly(vinylidene chloride-co-acrylonitrile); poly(caprolactone);poly(ethylene-co-vinyl acetate); poly(ethylene oxide); poly(butadiene);poly(epichlorohydrin); poly(styrene-co-butadiene); addition product ofsodium menthoxide to poly(pentafluorostyrene);(+)-isopinocampheol-derivatized poly(p-chloromethylstyrene);poly(fluorostyrene); and poly(styrene-co-isoprene) (Severin et al.(2000), supra).

For polymer film transducers, the film thicknesses are typically on theorder of 0.05 to 100 microns. The desired film thickness depends uponthe extent to which the film absorbs the image beam.

For polymer film transducers deposited on a substrate, transducercharacteristics that can vary and affect the measurements are:index-of-refraction, thickness, surface roughness, area, porosity, andtransducer-to-substrate bonding. Methods for depositing polymer filmdeposits on a substrate include dissolving the pulverized polymer with asuitable solvent and either manually depositing the solution on thesubstrate or using an inkjet printer. A grid of photoresist fabricatedvia photolithography on the substrate prior to deposition can be used toconstrain the polymer solution and produce manually depositedtransducers of more uniform area. Commercially available print heads canalso be used if the solvent is compatible with the print head materials.A polymer transducer may also be formed on a substrate by depositingpolymer solution on a Mylar® or Teflon® sheet, curing the polymersolution on the sheet, cutting the cured polymer and sheet to thedesired transducer dimensions, and joining the sheet side of thepolymer-sheet assembly to the substrate with an adhesive such as UVcured cement. However the polymer solution is deposited, the film can becured in a sealed chamber in an atmosphere saturated with the solvent inorder to improve the uniformity of the film thickness. Other methodsknown to the art for depositing polymer films may also be employed.

A vapor feeding system can be used to control the vapor environment influid communication with the transducer. The vapor feeding system canisolate the transducer environment from the environment surroundingother elements of the sensor system by using a controlled environment“chamber” surrounding the transducer(s). The “chamber” can seal to thetransducer substrate with an o-ring or by other means known to thoseskilled in the art.

One or more gas lines can be used to introduce pulses, “sniffs” or“breaths” of vapor into the “chamber.” The test vapor may be supplied tothe transducer either continuously or in “sniffs.” The vapor feedingsystem may also deliver a test vapor and a reference vapor to thetransducer alternately. As used herein, a “reference vapor” is a vaporselected for use in the measurement which may be the same or differentfrom the vapor to be tested or analyzed. A reference vapor may be avapor which does not induce polymer swelling such as noble gases orgases such as N₂, H₂, O₂ or CO₂. A reference vapor can also be a vaporthat is to be compared with a test vapor. In a perfume factory, forexample, the reference vapor may be the standard perfume odor. Thereference vapor can also be a vapor collected from the recent history ofthe environment or a vapor taken from another spatial region in thevicinity of the test vapor (for example, a test vapor passageway couldbe placed near the opening of a beaker while the reference vaporpassageway could be placed further away from the opening). A number ofstandard reference vapors can also be used and the vapor in questiontested against every one of the reference vapors. An electrically,pneumatically or manually actuated valve can be used to alternatelydeliver the test and reference vapors at a regular selected frequency.Typical exposure cycles can range from 100 ms to 2 s. Alternatively, thetest and reference vapors can be alternated without using a valve byusing syringes or by other means known to those skilled in the art.

A photorefractive element can be any photorefractive material suitablefor use with the devices and methods of invention. As used herein, aphotorefractive material is a material which has an index of refractionwhich depends on the applied electric field. The photoelectric effect isdescribed, for example, by Glass (A. M. Glass, (1978) OpticalEngineering, Vol. 17, p.470). Suitable photorefractive materials includephotorefractive crystals. Photorefractive crystals preferred fortwo-beam coupling devices include barium titanate, lithium niobate,strontium barium niobate (SBN) and several others known to those skilledin the art of photorefractive materials and devices. It is preferredthat the coupling strength (Γ) times the length of the medium (L) ishigher than about 1, and more preferred that it is on the order of 10 orso.

In the configuration shown in FIG. 1, the detector is placed on the pathof the image-carrying beam after the photorefractive crystal. To usemultiple detectors, the beam may be split or the detectors may bearranged in an array which mimics the array of transducers. Acharge-coupled device (CCD) camera, which acts as many detectors, orother non-CCD imaging array sensitive to the light beam can be used torecord the response pattern. In the two-beam coupling sensor systemdescribed above, the camera will only detect the light from activatedpolymer spots. A photodiode can be used to detect the beam intensity.

A control system can be used to synchronize the detector with the vaporfeeding system to increase the sensitivity of the sensor system. For avapor feeding system which uses an electrical switch to alternatebetween a test vapor and a reference vapor at a particular frequency,the control system can synchronize the sampling rate of the detectorwith the signal that drives the switch. In each “sniff” cycle fromreference vapor to test vapor to reference vapor, the expected systemresponse frequency is twice that of the cycle frequency. A lock-inamplifier can be used to lock in the sampling rate to the secondharmonic of the “sniffing” and to set a phase shift to allow some delayfor vapor flow, vapor diffusion, and the response of the photorefractivecrystal. This procedure can help improve the signal-to-noise ratio ofsmall vapor-induced signals.

Holographic optical novelty filter versions other than the two-beamcoupling version shown in FIG. I and described herein can also be used,including those that do not require an externally-supplied referencebeam. Two examples are those that make use of beam-fanning (amplifiedspontaneous scattering), and those that use self-pumpedphase-conjugation (Anderson and Feinberg, 1989, supra).

Embodiments of the invention also provide a method for the detection ofa change in concentration of a test vapor in an environment comprisingthe steps of: providing a transducer capable of absorbing the test vaporand thereby changing the transducer; exposing the transducer to the testvapor; and detecting the change in the transducer using dynamicholography, thereby detecting the change in concentration of the vapor.As used herein, “detecting the change in the transducer using dynamicholography” involves generating an interference pattern which containsinformation about the change in the transducer, generating a holographbased on the interference pattern using a dynamic holographic medium oran apparatus that replicates the functionality of a dynamic holographicmedium, and reading out the hologram generated.

The change in the transducer can be detected in several ways. In thetwo-beam coupling method described above, the coherent light sourceproduces a source beam which is split into an image beam and a referencebeam. The image beam interacts with the transducer. A hologram based onthe interference of the image and a reference beam is then generatedwithin the photorefractive element. The hologram contains informationabout the change in the transducer, and thus the change in test vaporconcentration. Reading out the hologram results in an interferencepattern between the image beam and a portion of the holographic readoutat a detector as has been described above. The interference patternmeasured at the detector can be used to determine the change in thetransducer and thus the change in test vapor concentration.

In another mode of operation, the source beam acts as a first image beamsince no reference beam is split off prior to interaction of the sourcebeam with the transducer. Instead, the first image beam is split afterit interacts with the transducer into a second image beam and a thirdimage beam. The second and third image beams interact to produce ahologram using either a photorefractive element or digital holography.The hologram can be read out to determine the change in test vaporconcentration.

In yet another mode of operation, the source beam again acts as an imagebeam since no reference beam is split off. After the image beaminteracts with the transducer, it is used to create a hologram inside aphotorefractive element. In this case, the hologram is based on theinteraction of the image beam and amplified scattered light from theimage beam (photorefractive fanout). The hologram can be read out todetermine the change in test vapor concentration.

More generally, the methods and devices of the invention can employ adynamic holographic medium. As used herein, dynamic holographic mediainclude photorefractive materials and equivalent media with which onecan nearly simultaneously perform real-time dynamic holography, butwhich do not undergo the specific physical mechanisms associated withthe photoelectric effect. These media include photosensitivethermoplastic films and other photosensitive media.

Alternatively, the photorefractive element or equivalent medium can beeliminated and the hologram created using digital holography. Whendigital holography is employed, the interference pattern is recorded ona spatial recording device, such as a CCD camera, photodiode array, orcomplementary metal-oxide semiconductor (CMOS) camera. An informationprocessing device, such as a computer or microporcessor can be used toprocess the spatial information recorded. In an embodiment of anolfactory sensor system using digital holography, the dynamicholographic medium and the detector in optical communication with thedynamic holographic medium are replaced by a spatial recording deviceand an information processing device. Digital holography techniques areknown to those skilled in the art.

In the methods of the invention which employ a reference beam and animage beam, the reference beam or the image beam can be phase modulatedto introduce an extra periodic relative phase variation between thereference beam and the image beam. Phase modulation can also be used ina setup where no reference beam is present and the image beam is splitafter it interacts with the transducer (Beam-fanning novelty filter withenhanced dynamic phase resolution), H. Rehn et al., (1995) AppliedOptics-OT, Vol.34 No.2, p.4907) Phase modulation can increase the signalto noise ratio of the detector signal. Phase modulation is a techniqueknown to the art, and is described, for example by Rehn et al. (1995).In the experimental two-beam setup shown in FIG. 1, phase modulation canbe accomplished by attaching a piezoelectric device to mirror (70) totranslate the mirror and thereby impose a periodic phase variation onthe reference beam. In a two-beam setup, the phase modulator can beplaced on either beam and can be located anywhere after the beamsplitter and before the photorefractive element or equivalent. In asingle-beam setup, the phase modulator can be located anywhere after thelight source and before the photorefractive element or equivalent. Othermethods of performing phase modulation and phase modulators known to theart, for example, using an elecro-optic modulator (EOM) can be used.Sine waves, square waves and other periodic functions may be used in thephase modulation methods of the present invention. Methods of theinvention employing phase modulation are capable of detection at partsper billion levels.

Embodiments of the invention also provide a method for determining theconcentration of a test vapor which is not necessarily changing. In thismethod, a reference vapor is alternately supplied with the vapor to betested. The change between the reference vapor and the test vaporcreates a change in the vapor environment seen by the transducer, whichcan be detected using the methods described above. The changes can bequantified and correlated to vapor concentration by means known in theart.

Embodiments of the invention further provide a method for detection of achange in concentration of a plurality of test vapors in an environmentcomprising the steps of: providing a plurality of transducers eachcapable of absorbing a test vapor and thereby changing the transducer,wherein the transducers are selected so that at least one separatetransducer absorbs each of the test vapors; and detecting the change inthe transducers using dynamic holography, and analyzing this change,thereby detecting the change in concentration of the test vapors.

Embodiments of the invention also provide a method for the detection ofa change in concentration of a test absorbent in a liquid environmentcomprising the steps of: providing a transducer capable of absorbing thetest absorbant and thereby changing the transducer; exposing thetransducer to the test absorbant; and detecting the change in thetransducer using dynamic holography, thereby detecting the change inconcentration of the absorbant. The change in the test absorbantconcentration can cause changes in the transducer's dimensions, thetransducer's index of refraction and/or other changes that can bedetected optically using dynamic holography.

In the methods of the invention, the change in the transducer uponexposure to the test vapor or test absorbant may be any change that canbe detected optically using dynamic holography. For example, thetransducer may undergo a change in its dimensions and/or index ofrefraction.

EXAMPLE 1 Fabrication of Transducer Arrays

Calibration Array

An array of 16 poly(N-vinylpyrrolidone) transducers, which absorb waterand ethanol, was fabricated on a single glass slide. The transducerswere fabricated using a syringe to manually deposit the polymer solutionon the slide. Water was used as the solvent. The diameter of eachcircular transducer was approximately 0.4 mm, which was read from theimage displayed on the CCD camera.

2 by 2 Array

A 2 by 2 transducer array with two types of polymers,poly(N-vinylpyrrolidone) and poly(ethylene-co-vinyl acetate) wasfabricated on a single glass slide. Two transducers were fabricated ofpoly(N-vinylpyrrolidone), which absorbs water and ethanol, and two werefabricated of poly(ethylene-co-vinyl acetate), which absorbs hexane. Thetransducers were fabricated using a syringe to manually deposit thepolymer solution on the slide. Epoxy ethanol was used as the solvent forpoly(N-vinylpyrrolidone) while toluene was used as the solvent forpoly(ethylene-co-vinyl acetate). The diameter of each circulartransducer was approximately 0.7 mm.

EXAMPLE 2 Two-Beam Coupled Sensor System

A two-beam coupled sensor system similar to that shown in FIG. 1 hasbeen constructed and its operation demonstrated. The system wasapproximately 14 cm×11 cm. The coherent light source was a solid statedouble frequency laser with 532 nm selected as the operating wavelength(Crystal Laser). This laser had a power of 75 mW and an initial beamdiameter of about 0.8-1.5 mm. Beam shaping elements were used to expandthe beam to a 5 mm by 5 mm square beam. The transducers were fabricatedon glass slides as described above. The system as described is capableof analyzing a transducer array of greater than 16 elements and shouldbe capable of analyzing a transducer array of 100 elements. The vaporfeeding system isolated the transducer environment from the environmentsurrounding other elements of the sensor system by using a controlledenvironment “chamber” surrounding the transducer(s). An electric valvealternately delivered pulses of a test vapor and a reference vapor intothe “chamber” at a “sniff” cycle frequency of approximately 1.75 Hz. Thephotorefractive element was a barium titanate crystal with a couplingconstant (Γ) *L of approximately 6.2. Both a CCD camera (dynamic rangeapproximately 70 dB) and a photodiode (dynamic range approximately 100dB) were used as detectors. Both the camera and the photodiode were lownoise. To minimize mechanical noise, the interferometer was isolatedfrom other parts of the system which could generate mechanical vibration(e.g., valves, pumps). One method of isolating the interferometer is toplace it in an enclosure supported by rubber dampers or other types ofshock absorbing materials or devices known to those skilled in the art.

The system was calibrated to determine the relation between the phaseshift of the beam and the output power of the system. To calibrate thesystem, a piezo-driven mirror was put on the reference beam. Thetranslation of the mirror modulates the phase of the beam. The smallestdetectable translation with the system was 0.1 nm for an integral timeof about 10 seconds and 0.45 nm for an integral time of about 1 second.

The concentration of the test vapors was also calibrated with the outputpower of the system using the calibration transducer array describedabove. FIG. 4 shows the relationship between the detector reading andconcentration for ethanol vapor. The smallest ethanol vaporconcentration detected with this calibration transducer array was 40ppm. The smallest water vapor concentration detected with the same arraywas 41 ppm. The detection limit can also be presented in normalizedterms as ppm mm²/sqrt(Hz). An improved sensitivity for water vapor of8.3 ppm mm²/sqrt(Hz) was obtained by using a transducer of improvedsurface quality obtained by curing the polymer in a sealed chamber asdescribed above.

Pattern recognition was tested using the 2 by 2 array described above.FIGS. 5A-5C show the response pattern to ethanol (5A), the responsepattern to hexane (5B), and the response pattern to a mixture of ethanoland hexane (5C). In FIGS. 5A-5C, the response of both of the polymers inFIG. 5C is weaker than that in FIG. 5A or FIG. 5B because theconcentration of each vapor is lower in the tested mixture.

FIG. 6 shows the response of the sensor system to changes inconcentration of the vapor environment. In FIG. 6, high voltage levelsof the “sniff” control signal represent the phase when the system“sniffs” the reference vapor and low voltage levels represent the phasewhen the system “sniffs” the test vapor. The peaks at the front edge ofthe “sniff” control signal are much higher than those at the rear edge.This occurs because the gradient of the vapor concentration is largerwhen the reference vapor goes into the system. The magnitude of theresponse drops with a decrease in the vapor concentration.

The relationship between the minimum detectable signal and the area ofthe transducer was investigated. The poly(N-vinylpyrrolidone)transducers were between 10 and 20 microns thick. The transducers werefabricated on glass slides, with each slide having different numbers oftransducers. The transducers were fabricated using the manual depositiontechniques described above. FIG. 7 shows the relation between thesensitivity and the area of the transducer. From the figure, therelationship appears to be close to linear. The integral time for themeasurement was one second.

EXAMPLE 3 Two-Beam Coupled Sensor System with Phase Modulation ofReference Beam

The two-beam coupled sensor system of Example 2 was modified byattaching a piezoelectric device to drive mirror (70) thereby phasemodulating the reference beam. The modulation signal on the referencebeam had an amplitude of 1.3 radian (110 nm) and a frequency of 6.2 Hz.The vapor signal sniff-cycle frequency was approximately 1.4 Hz.

FIG. 8 shows detector signal as a function of ethanol vaporconcentration for poly(N-vinyl pyrollidone) transducer array havingsimilar thicknesses and areas to those described in Example 1. Theintegral time for the measurement was 5 seconds. The detector signal wasobserved at the sniff -cycle frequency (the detector was synchronizedwith the sniff-cycle frequency). The normalized sensitivity for ethanolvapor was approximately 60 ppb mm²/sqrt.

Those of ordinary skill in the art will appreciate the existence ofequivalents of device elements, method steps, and materials, all knownfunctional equivalents of which are encompassed by the invention. Allreferences cited herein are hereby incorporated by reference to theextent not inconsistent with the disclosure herewith.

1. A method for the detection of a change in concentration of a testvapor in an environment comprising the steps of: a) providing atransducer capable of absorbing the test vapor and thereby changing thetransducer; b) exposing the transducer to the test vapor; and c)detecting the change in the transducer using dynamic holography, therebydetecting the change in concentration of the test vapor.
 2. The methodof claim 1, wherein the change in the transducer is detected by: a)producing a coherent light source beam; b) dividing the source beam intoan image beam and a reference beam; c) positioning at least onetransducer so that it interacts with the image beam, wherein thetransducer is capable of absorbing the test vapor, thereby changing thetransducer; d) after the image beam has interacted with the transducer,combining the image beam and the reference beam, thereby generating aninterference pattern; e) using dynamic holography to produce a hologrambased on the interference pattern; and f) reading out the hologram,thereby detecting the change in concentration of the test vapor.
 3. Themethod of claim 1, wherein the change in the transducer is detected by:a) producing a coherent light source beam which is a first image beam;b) positioning at least one transducer so that it interacts with thefirst image beam, wherein the transducer is capable of absorbing thetest vapor, thereby changing the transducer; c) after the first imagebeam has interacted with the transducer, dividing the first image beaminto a second image beam and a third image beam; d) combining the secondimage beam and the third image beam, thereby generating an interferencepattern; e) using dynamic holography to produce a hologram based on theinterference pattern; and f) reading out the hologram, thereby detectingthe change in concentration of the test vapor.
 4. The method of claim 1,wherein the change in the transducer is detected by: a) producing acoherent light source beam which is an image beam; b) positioning atleast one transducer so that it interacts with the image beam, whereinthe transducer is capable of absorbing the test vapor, thereby changingthe transducer; c) after the image beam has interacted with thetransducer, using dynamic holography to generate a hologram within aphotorefractive element, the hologram being based on the interaction ofthe image beam and amplified scattered light from the image beam; and d)reading out the hologram, thereby detecting the change in concentrationof the test vapor.
 5. The method of claim 1, comprising creating ahologram within a photorefractive element using dynamic holography. 6.The method of claim 1, comprising digitally creating a hologram usingdynamic holography.
 7. The method of claim 5, further comprising readingout the hologram, and analyzing the holographic readout at a detector.8. The method of claim 1, further comprising the step of alternatelyexposing the test vapor and a reference vapor to the transducer.
 9. Themethod of claim 8, further comprising creating a hologram within aphotorefractive element using dynamic holography, reading out thehologram, analyzing the holographic readout at a detector, andsynchronizing the detector with the rate at which the test vapor and areference vapor are alternated.
 10. The method of claim 1, wherein thetransducer is a polymer film supported on a substrate.
 11. The method ofclaim 1 further comprising exposing a plurality of transducers to thetest vapor.
 12. The method of claim 1 wherein the change inconcentration is detected in less than about 5 seconds.
 13. The methodof claim 1 wherein the change in concentration is detected in less thanabout 2 seconds.
 14. The method of claim 1 wherein the dimensions of thetransducer change when it is exposed to the test vapor.
 15. The methodof claim 1 wherein the index of refraction of the transducer changeswhen it is exposed to the test vapor.
 16. The method of claim 1 whereinthe dimensions and the index of refraction of the transducer change whenit is exposed to the test vapor.
 17. A method for the detection of achange in concentration of a plurality of test vapors in an environmentcomprising the steps of: a) providing a plurality of transducers eachcapable of absorbing a test vapor and thereby changing the transducer,wherein the transducers are selected so that at least one separatetransducer absorbs each of the test vapors; b) exposing the transducersto the test vapors; and c) detecting the change in the transducers usingdynamic holography, thereby detecting the change in concentration of thetest vapors.
 18. A method for the determination of the concentration ofa test vapor in an environment comprising the steps of: a) providing atransducer capable of absorbing the test vapor and thereby changing thetransducer; b) alternately delivering the test vapor and a referencevapor to the transducer; and c) detecting the change in the transducerusing dynamic holography when the vapor and the reference vapor arealternated, thereby detecting the concentration of the test vapor. 19.An olfactory sensor system for detecting changes in test vaporconcentration in an environment comprising: a) a coherent light sourcecapable of producing a beam of light; b) a transducer in opticalcommunication with the light source, in fluid communication with theenvironment and capable of responding to a change in test vaporconcentration; c) a dynamic holographic medium in optical communicationwith the transducer; and d) a detector in optical communication with thedynamic holographic medium, wherein at least part of the beam of lightpasses from the light source to the transducer, from the transducer tothe dynamic holographic medium, and from the dynamic holographic mediumto the detector.
 20. The sensor system of claim 19 wherein said detectorhas a sampling rate and further comprising a vapor feeding systemcapable of alternately delivering the test vapor and a reference vaporto the transducer at a rate of alternation and control equipment capableof synchronizing the sampling rate of the detector with the rate ofalternation.
 21. The sensor system of claim 20 wherein the controlequipment is a lock-in amplifier.
 22. The sensor system of claim 19wherein the detector comprises a CCD camera and a photodiode.
 23. Thesensor system of claim 19 further comprising analysis equipment inelectrical communication with the detector.
 24. The sensor system ofclaim 19 which is portable.
 25. The sensor system of claim 19 whereinthe transducer is a polymer film.
 26. The sensor system of claim 19comprising a plurality of transducers.
 27. The sensor system of claim 26wherein at least two of the transducers are different in composition.28. An olfactory sensor system for detecting changes in test vaporconcentration in an environment comprising a) at least one transducerlocated on a substrate, wherein the transducer is in fluid communicationwith the environment and capable of responding to a change in test vaporconcentration; b) a vapor feeding system; c) an interferometer systemcomprising i) a coherent light source capable of producing a source beamof light, ii) a splitter for splitting the source beam into an imagebeam and a reference beam, iii) at least one image-beam directingelement for directing the image beam to the transducer; iv) apolarization-modifying element placed in the path of the image beamafter it interacts with the transducer; v) at least one reference-beamdirecting element for directing the reference beam so that it may becombined with the image beam after the image beam interacts with thetransducer; vi) a polarization control element in the path of thereference beam; d) a photorefractive element placed so that the imagebeam and the reference beam interfere within the photorefractiveelement, the photorefractive element being capable of producing ahologram; and e) at least one detector in optical communication with thephotorefractive element.
 29. The sensor system of claim 28 wherein thedetector has a sampling rate and the vapor feeding system is capable ofalternately delivering a test vapor and a reference vapor to thetransducer at a rate of alternation and further comprising controlequipment capable of synchronizing the sampling rate of the detectorwith the rate of alternation.
 30. The sensor of claim 29 wherein thecontrol equipment comprises a lock-in amplifier.
 31. The sensor of claim28 further comprising analysis equipment in electrical communicationwith the detector.
 32. The sensor system of claim 28 wherein thedetector comprises a CCD camera and a photodiode.
 33. The method ofclaim 2 wherein the reference beam is phase modulated.
 34. The sensor ofclaim 28 further comprising a phase modulator placed in the path of thereference beam after the splitter and before the photorefractiveelement.